TOPIC 5 What Channel Is That?

Light isn’t the only kind of radiation coming from the stars. In the late nineteenth century, scientists found out that light is just one form of electromagnetic radiation. Other forms include waves, waves (heat), waves, X rays, and gamma rays. Look at Figure 5.33. This shows the entire spectrum of electromagnetic radiation. Notice that light waves occupy only a small portion of the entire spec- trum. In Topic 5, you will focus on radio waves and how astronomers use them to learn about the composition of stars — radio .

infrared visible ultraviolet radio waves microwaves X-rays gamma rays radiation light radiation Frequency 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 (hertz) Wavelength 5 4 3 2 -1 -2 -3 -4 -5 -6 -7 -8 -9 -10 -11 -12 -13 -14 -15 10 10 10 10 10 1 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 (metres)

Figure 5.33 The electromagnetic spectrum.

Find Out Give Me Some Static! Lightning bolts emit radio waves. In fact, any 3. Bring the radio close to where you make a static electric sparks do the same. In this activ- static spark. Listen on the radio for the stat- ity, you will make static sparks and try to ic electricity to “broadcast” on the radio. detect radio waves from them. 4. Repeat step 3 several times but at different Materials frequencies (between different stations). small A.M./F.M. radio Record your findings in your notebook. wool plastic 5. Switch to F.M. and repeat step 3 at differ- fur or hair ent frequencies. Performing and Recording Procedure What Did You Find Out? Analyzing and Interpreting Communication and Teamwork 1. Could your radio pick up the static on 1. Experiment to find a way to get a fairly A.M.? On F.M.? good static shock with the materials provided. 2. Why do you think that lightning causes problems for radio listeners? 2. Tune the radio between two A.M. stations. You should hear a hissing sound. 3. Name three things in your house that might make static shocks. Do they cause radio interference?

What Channel Is That? • MHR 393 Radio Telescopes Telephone calls used to be transmitted by radio

Astronomers were able waves. The reception was to adapt radar equip- poor, and people had to ment used in World shout over hiss and noise War II to become the on the line. In 1932, Karl first radio telescopes. , an engineer work- This was quite simple ing for Bell Telephone because radar equip- Laboratories, was given ment emits and detects radio waves. the job of tracking down When radar equipment the radio emissions that emits radio waves, the were interfering with these waves reflect off an communications. Jansky object allowing the built a radio antenna. distance to the reflect- Using this antenna, he ing object to be calculated. However, learned to identify radio astronomical radars do emissions that rose and set not emit radio waves with the Sun, planets, and because there are no stars. From these observa- Figure 5.34 The world’s first (inset) by objects reflecting tions, he concluded that Karl Jansky, marked a big moment in science as was them. Instead, the these sources Galileo’s optical telescope 300 years before. astronomical radars came from space. (the radio telescopes) simply detect radio , a radio engineer and amateur astronomer, explored waves emitted from Jansky’s discovery further. Reber built a radio dish (a radio telescope) and objects in space. “listened” to the sky during the 1930s. He discovered that the strongest radio waves came from particular places in the sky. He thought there must be some radio objects in space that were responsible for these emissions. “Listening” to the stars through Reber’s radio telescope would be like Reber found that the tuning a radio between channels. Reber would hear hissing static. The object outside of the hiss would become louder when he tuned in to an area in space that was solar system producing giving off large amounts of radio waves — the bright radio objects. the strongest radio waves (called the Bigger Radio Telescopes brightest radio object) is the centre of our Recall that the resolving power of an optical telescope relates to the fine- — the Milky Way. The ness of detail it can image. The wavelength of the light is one factor for Sun is the brightest of the resolving power — the smaller the wavelength, the better the resolv- all the radio objects in ing power. Radio waves have wavelengths that are millions of times the sky, and Jupiter is longer than light waves. This means radio waves provide images with less the second brightest. resolution than light waves. However, radio waves penetrate dust clouds in the galaxy where visible light stops. So radio telescopes gave astronomers information about the universe that they never had before.

394 MHR • Space Exploration “Seeing” Radio Waves You may wonder how produces images of radio sources. Radio telescopes cannot “see” radio sources. In the early days of professional radio astronomy, the movement of dials and needles monitored the incoming radio waves. The needle is similar to the kind you see in an ammeter or volt- meter. Astronomers then graphed the data. Today, computers store the same data and false colour it to produce images of the radio waves The colour is coded to the strength of the sig- nal. Usually, blues are for low intensities and as the signal gets stronger, the colours go through greens, yellows, reds, and finally to whites.

Optical Connections Radio astronomers wanted to identify their strong radio sources with objects they had seen with optical telescopes. Figure 5.35 The collecting dishes of the This was so they could be sure just what objects were emitting first radio telescopes were huge radio waves. This was impossible at first because the radio compared to optical telescopes. Parke’s radio telescope dish in New South Wales, images had such low resolution. As the radio telescopes Australia, is 64 m in diameter. improved, astronomers could make these optical connections. For one example of an optical connection, compare Figure 5.36A and Figure 5.36B.

Figure 5.36A This is a visible light image of , an active radio galaxy, 16 million light-years away. (Any galaxy that emits strongly in radio waves is called an active galaxy.) You see the central nucleus of the galaxy with a lane of dust across it.

The Lovell radio telescope, in Britain, was built in 1957. Radio telescopes Figure 5.36B This is a radio image of Centaurus A. The work just as well through telescope scanned the sky near the galaxy, and the clouds as through clear skies, so rainy places, like radio waves detected were recorded in a computer. The Britain, are fine for radio telescopes! computer then converted the strength of the signals to colours, which produced the image shown here. The visible light image covers only the white section of the central region of the radio image. The radio images show how the energy emitted from the galaxy extends beyond what we can see in visible light.

What Channel Is That? • MHR 395 Connecting Radio Telescopes Astronomers improved radio images by connecting telescopes. If two radio telescopes (and more recently optical telescopes) are separated by some distance, but connected electronically, their signals can be com- bined using a computer. The resulting images are as good as if one telescope were used that was as big as the distance between Figure 5.37A The is a the two. This method is Y-shaped array of 27 identical, 25 m dishes called . on railroad tracks. Electric cables connect the It’s like seeing with many dishes. Each of the three sections of the Y is eyes instead of one. The about 20 km long. The VLA has a resolution so fine that the centimetre marks on a ruler most accurate set of con- could be seen 5 km away — if the ruler were nected telescopes in the broadcasting radio waves! world is the Very Large Array (VLA) in New Mexico, U.S. Figure 5.37B A single dish from the Very Large Array.

Radio Telescopes Bigger Than Earth Improvements in computers and the precision of modern clocks have enabled radio astronomers to connect their telescopes without wires. This is called very long baseline Figure 5.38 The extra interferometry (VLBI). VLBI resolution offered by the produces images 100 times as VLA was used to produce an image of the central detailed as the largest optical white region of Figure telescopes that exist today. 5.36B. The scale of this Astronomers combine signals image is similar to the from any (and as many as they scale of the visible light Figure 5.39 Canadian astronomers are participating in image in Figure 5.36A. want) radio telescopes in the an international VLBI project called VSOP (VLBI Space Many astronomers believe world. Astronomers record Observatory Program). This project uses a radio dish in that a black hole is drawing each telescope’s signal with space and an array of ground-based telescopes, which material into it at the centre timing marks. The signals are simulates a single dish twice Earth’s diameter! of this galaxy. The energy Astronomers at the University of Calgary calibrate and is given off by material transferred to computer disks, image some of the data for this program. This before it disappears into loaded onto a central comput- telescope has been able to image objects over 13 billion the black hole. er, and combined to form one light-years from Earth. image. In theory, astronomers can create a telescope as big as Earth using this technique.

396 MHR • Space Exploration As a child, Betsy Barton watched television astronomy shows and read all she could about astronomy. She began her career with a lifelong love for physics and math. After receiving an undergraduate degree in this area, she discovered her love for astronomy. Her Ph.D. thesis project explored galaxy interactions, and she enjoyed it so much that she has car- ried on this research at the National Research Council. Now, she’s a professional galaxy gazer in her position as a research associate in Victoria. Betsy is particularly interested in what happens when two come close to each oth- er and the effects they have on one another. As galaxies near each other — that is, come less than about 150 000 light-years from one another — they can be reshaped by the oth- er’s gravitational pull. Gas from the outer edges of galaxies can then funnel to the centre, forming new stars. This phenomenon takes place over millions of years, of course, so Betsy can’t watch it happen. She can only infer that this is happening because so many galaxies that have oth- er galaxies close by also have new stars in their centres. She has been using a group of about 500 galaxies to study the effects of galaxy interaction. To study these galaxies, she Betsy Barton gathers optical images as well as radio observations collected by the Very Large Array telescope in New Mexico. Are you interested in astronomy as a career? • Talk to university or college professors to find out about careers in astronomy. • Visit a planetarium to learn more about space. • Conduct your own research on such topics as constellations, supernovae, galaxies, or comets and meteors.

TOPIC 5 Review 1. How did Karl Jansky know that some radio waves come from space?

2. Why are radio telescopes built so much larger than optical telescopes?

3. Explain the technique of interferometry.

4. Describe how very long base line interferometry works.

5. Thinking Critically If you wanted to build a radio telescope, would you build it in a country with lots of rain, sunshine, or both? Would either type of location affect the telescope’s ability to make accurate observa- tions? Explain your answer.

www.mcgrawhill.ca/links/sciencefocus9 NASA has a program to develop and place telescopes into orbit that use all of the electromagnetic windows — from radio waves to gamma rays. Find out about one telescope program that is in opera- tion. Go to the web site above, and click on Web Links to find out where to go next. Make a poster or write a brief report detailing your findings.

What Channel Is That? • MHR 397 Wrap-up TOPICS 3-5

If you need to check an item, Topic numbers are provided in brackets below. Key Terms spectrum spectral analysis astronomical unit interferometry spectroscope Doppler effect light-year very long baseline spectral lines red shifted electromagnetic radiation interferometry spectroscopy adaptive optics radio astronomy diffraction grating triangulation radio object

Reviewing Key Terms Understanding Key Concepts 1. In your notebook, match the description in 2. What is a spectroscope? (3) column A with the correct term in column B 3. How is an emission spectrum produced? How AB is an absorption spectrum produced? (3) • a series of closely • adaptive optics (4) spaced lines that splits 4. How can the spectrum of a star tell us if it is light into a spectrum approaching Earth? (3) • all the various kinds of • Doppler effect (3) 5. What kind of material can radio waves pene- radiant energy trate that light waves cannot? (5) • telescope technology • radio astronomy (5) that removes the 6. Thinking Critically How can larger tele- effects of the atmosphere scopes give astronomers a more precise picture of the distances to the nearest stars? • when images from • light-year (4) two radio telescopes (4) are combined using 7. Thinking Critically Television signals are a computer radio waves which travel at the . • using radio waves to • spectroscopy (3) The first commercial TV broadcasts began learn about the stars about 1950. How far have these radio waves • the distance that light • diffraction grating (3) travelled in light-years? (5) travels in one year 8. Apply In one year, light travels about • one method to • electromagnetic radiation (5) measure the distance 63 240 AU. Using this scale, look at the illus- to the stars tration and state, in light years, how far away star A is from the Sun. (4) • helps to measure the • interferometry (5) speed and direction of stars • the scientific study of • triangulation (4) spectra distance: 543 000 AU

star A the Sun

398 MHR • Space Exploration